This specification relates to assessing entropy in a cryptography system. Cryptography systems enable secure communication over public channels. For example, in public-key cryptography systems, a message sender encrypts a plaintext message using an encryption algorithm, and transmits the encrypted message to a recipient. The recipient can use a secret key value to recover the plaintext message from the encrypted message. In some cryptography systems, the entropy of the secret key value makes the cryptography system robust against certain types of attacks.